By definition a trilogy is " a series or group of three plays, novels, operas, etc., that, although individually complete, are closely related in theme, sequence, or the like. So they qualify. I don't thing Ford regarded them as a trilogy.
Yep. Thematically they all share pretty much the same qualities, but by today's definition most audiences would not look at those and call that a trilogy. It would be like saying Tarantino is working towards a trilogy after doing Hateful Eight and Django. So while the technical definition is satisfied, a new fan unfamiliar with these films might buy the old 'Cavalry trilogy' DVD box set expecting a continuous narrative and be surprised that is not the case.
I just wonder why Ford did not want to do a character study over those three films and keep some continuity since the treatment was so close between those characters. All three films are brilliant, so there's no questioning their individual merits. It's just one of those odd anomalies in Ford's portfolio that is referenced in a way he never intended. I've always found that interesting. Great point adding Horse Soldiers to that list Ethan! Another personal favorite.